Electrodeposition techniques are known in the art. In general, organic coatings can be applied to conductive substrates such as metals by electrodeposition. The invention relates to improved electrocoating bath compositions and methods for electrodepositing a coating on the interior and exterior portions of electrically conductive surfaces such as metal articles. An aqueous media containing the organic coating to be applied is formed and is commonly referred to as an e-coat bath. The aqueous media typically contains from about 5% to about 30% solids.
An organic polymer is dispersed throughout the bath and may be positively or negatively charged. The coating of the substrate is accomplished by immersing the substrate in the aqueous media and applying a current to the substrate. The charged polymers are attracted to the material to be coated, which has the opposite charge. There the organic particles are deposited onto the part where they coalesce forming a coating. The part is removed from the e-coat bath, rinsed and the coating is cured. Curing the organic coating to form a final film aids in making the coating tough, resistant to water and organic solvents, etc.
Electrophoretic deposition systems are characterized as either anodic or cathodic thereby indicating the function of the object to be coated. Where the object to be coated is the cathode the system is referred to as a cathodic system. Similarly where the part to be coated is the anode the system is referred to as an anodic system.
The thickness of the layer of electrodeposited material is determined by voltage, coulombic efficiency, and other process factors. Coating takes place over the exposed substrate surface until the deposited film forms an insulating layer which greatly diminishes the flow of current thereby inhibiting and eventually limiting further growth in thickness. In order to obtain increased film thickness, the rate of deposition allowed by direct current flow must exceed the rate of dissolution due to the tendency of the deposited material to redissolve in or reenter the aqueous medium.
The art recognizes two distinct water-based dispersions, one is a solubilized dispersion and the other a nonsolubilized dispersion. The mechanisms of coating for these systems are different. In the solubilized system the organic coating is usually a water-insoluble organic resin chemically modified to contain solubilizing groups. In the case of an anionic resin the solubilizing groups are commonly a carboxylic acid. The solubilizing groups for cathodic resins are commonly amines. Thus, in the solubilized system the resin has been ionized. In contrast, the nonsolubilized systems (emulsions) are basically dispersed hydrophobic resins. In the nonsolubilized system bath stability is achieved with the use of emulsifying agents, ions, or ionizable species which are absorbed on the particle.
The present invention relates to solubilized electrocoating systems. Solubilized electrocoating systems have several disadvantages one of which is that the film building is self-limiting and the achievable thickness usually does not exceed 0.50 to 2.0 mils. Uniform coatings of acceptable quality for commercial purposes have been limited to a maximum thickness of about 2.0 mils. These relatively thin films do not have desirable properties for many applications. Thus, there has been a need in the art to provide materials and methods to achieve greater film thickness by aqueous electrodeposition techniques. The latter process has inherent advantages including economical labor costs, efficient material usage, and fast processing times. Coating thicknesses above 2.0 mils would be expected to enhance film wear under abrasive conditions, and to provide increased corrosion protection, as well as, resistance to other environmental conditions affecting metallic substrates.
It has now been found that according to this invention, the thickness limitations of prior aqueous solubilized electrocoating systems have been overcome. Using conventional operating conditions, coatings of greater thickness for a particular e-coat bath can be achieved, and in some cases coatings of up to about 8 mils can be obtained employing the electrocoating methods and materials of this invention.
The thicker coatings obtained by the present invention can be obtained without adversely affecting the quality of the deposited film.